Since our counter only counts 0, 1, 2 and 3, "11" is
the largest binary number displayed by this circuit. Ask your math teacher
to recommend some books about the binary number system. See
Binary Number System

We've already mentioned that binary numbers are used
to store numbers in computers. Sometimes you'll see the words octal and
hexadecimal used to describe the numbers used in microcomputer systems. Try
looking up the meanings for those two words in a dictionary.